1. Field of the Invention
The present invention relates to a laser irradiation apparatus and a method for manufacturing a semiconductor device with the use of the laser irradiation apparatus.
2. Description of the Related Art
In recent years, a technique to form a thin film transistor (hereinafter referred to as a TFT) over a substrate has made great progress, and application development to an active matrix display device has been advanced. Particularly, a TFT formed using a poly-crystalline semiconductor film is superior in field-effect mobility to a TFT formed using a conventional amorphous semiconductor film, and therefore high-speed operation becomes possible when the TFT is formed using the poly-crystalline semiconductor film. For this reason, a circuit for driving a pixel, which has been mounted by an external IC chip, can be formed integrally with the pixel over the same substrate by the TFT.
The poly-crystalline semiconductor film suitable for manufacturing a TFT is obtained by crystallizing an amorphous semiconductor film. To crystallize the amorphous semiconductor film, a laser annealing method is generally employed. The laser annealing is more preferable than a general thermal annealing that requires a temperature as high as 600° C. or more. This is because an inexpensive glass substrate, which is often employed as a substrate of TFT, is inferior in heat resistance and is easy to change in shape due to the heat. That is to say, the laser annealing has advantages that the processing time can be shortened to a large degree compared with another annealing method using radiation heat or conduction heat and that a semiconductor substrate or a semiconductor film on a substrate can be heated selectively and locally so that the substrate is hardly damaged thermally. Therefore, the laser annealing method is widely used to crystallize the amorphous semiconductor film formed over the glass substrate.
It is noted that the laser annealing method described herein includes the technique to recrystallize an amorphous layer or a damaged layer formed in the semiconductor substrate or the semiconductor film and the technique to crystallize an amorphous semiconductor film formed over the substrate. In addition, the technique to flatten or modify the surface of the semiconductor substrate or the semiconductor film is also included.
In the laser oscillators, there are a pulsed laser oscillator and a continuous wave laser oscillator according to the oscillation method. The laser annealing often uses a laser beam oscillated from the pulsed laser oscillator typified by an excimer laser. This is because the pulsed laser oscillator has an output power per unit time that is approximately three to six digits higher than that of the continuous wave laser oscillator. It is preferable to perform the laser annealing in such a way that a beam spot (a region irradiated by the laser beam in a surface of a processing object) is shaped into a square having a length of several cm on a side or into a line having a length of 100 mm or more through an optical system and that the beam spot is moved relative to the irradiated surface because this method provides high productivity and is superior industrially. (For example, refer to a patent document 1) For this reason, the pulsed laser oscillator is mainly used to crystallize the semiconductor film. It is noted that a laser beam having a rectangular shape on the irradiated surface is referred to as a rectangular beam, and a laser beam having a linear shape on the irradiated surface is referred to as a linear beam.
[Patent Document1] Japanese Patent Laid-Open No. H08-088196
It is noted that the term of linear herein used does not mean a line in a strict sense but means a rectangle having a large aspect ratio (or an oblong). For example, the rectangle having an aspect ratio of 2 or more (preferable in the range of 10 to 10000) is referred to as the line. It is noted that the linear is still included in the rectangular.
Since the laser beam emitted from the laser oscillator generally has Gaussian distribution in which the intensity of the laser beam is attenuated from the center toward the end portion, it is necessary to homogenize the intensity distribution of the laser beam on the irradiated surface in order to perform the homogeneous laser annealing. In recent years, in order to homogenize the intensity distribution, a method is often employed in which a cylindrical lens array is used to divide the laser beam in a predetermined direction and the divided laser beams are superposed in the same surface. According to this method, a rectangular beam can be formed which has a length of 300 mm or more in a direction of its long side and a length of 1 mm or less in a direction of its short side, and therefore it has become possible to perform the laser annealing efficiently to the semiconductor film formed over a large substrate.
A general method for homogenizing the intensity distribution of the laser beam emitted from the laser oscillator by a cylindrical lens array is explained with reference to FIGS. 6A and 6B. FIGS. 6A and 6B show simply the structure in which a cylindrical lens array and a cylindrical lens are used in combination to form a linear beam on an irradiated surface. It is noted that the linear beam formed on an irradiated surface 608 in FIG. 6A has a short side in a direction that is parallel to the paper. In a side view of FIG. 6A, a laser oscillator 601 is a XeCl excimer laser. A laser beam emitted from the laser oscillator 601 is incident into a cylindrical lens array 602 and divided into four. After that, the divided beams are combined once into one beam spot by a cylindrical lens 604 to form an image having homogeneous intensity distribution. Then, the beam spots separated again are reflected by a mirror 606 and condensed into one beam spot again by a cylindrical lens 607. After that, the laser beam is irradiated to an irradiated surface 608. Thus, the linear beam having the intensity distribution homogenized in a direction of its short side is formed on the irradiated surface 608 and the length thereof in the direction of its short side is determined.
Next, a top view of FIG. 6B is explained. The beam spot of the laser beam emitted from the laser oscillator 601 is divided into three by a cylindrical lens array 603. After that, the beam spots divided into three are combined into one beam spot on the irradiated surface 608 by a cylindrical lens 605. A dotted line behind the mirror 606 shows a correct optical path and correct positions of the lens and the irradiated surface in the case not providing the mirror 606. This homogenizes the intensity distribution of the beam spot shaped into linear in the direction of its long side and determines the length thereof in the direction of the long side.
The linear beam spot transformed by the above structure is irradiated as being overlapped in such a way that the linear beam spot is displaced gradually in the direction of the short side of the linear beam spot. With such irradiation performed, the laser annealing can be performed to the whole surface of the non-single crystal silicon film so as to crystallize it or to enhance its crystallinity for example.
However, when the cylindrical lens transfers the image having homogeneous intensity distribution to the irradiated surface, the homogeneity of the intensity distribution of the beam spot formed on the irradiated surface is adversely affected by the aberration. FIG. 2 is an enlarged view of a structure in which the intensity distribution of the linear beam is homogenized in a direction of its short side by a cylindrical lens array and two cylindrical lenses. The laser beams divided into four by a cylindrical lens array 201 are combined into one beam spot by a cylindrical lens 202. The intensity distribution is homogenized in the combined beam spot. This beam spot is transferred to an irradiated surface by a cylindrical lens 203, which is also referred to as an imaging optical system. On this occasion, in consequence of the aberration depending on the incident height of the respective rays in the laser beam incident into the cylindrical lens 203, the rays are focused at the different points as shown in FIG. 2. This makes it difficult to enlarge a region having homogeneous intensity distribution in the whole area of the beam spot. As a result, when the laser annealing process is performed using the semiconductor film as the irradiated surface, the aberration causes a problem of low throughput because the region that can be annealed by the beam spot having the homogeneous intensity distribution is not large.